The statements in this section merely provide background information related to the present disclosure and may not constitute prior art.
Layered heaters are typically used in applications where space is limited, when heat output needs vary across a surface, where rapid thermal response is desirous, or in ultra-clean applications where moisture or other contaminants can migrate into conventional heaters. A layered heater generally comprises layers of different materials, namely, a dielectric and a resistive material, which are applied to a substrate. The dielectric material is applied first to the substrate and provides electrical isolation between the substrate and the electrically-live resistive material and also reduces current leakage to ground during operation. The resistive material is applied to the dielectric material in a predetermined pattern and provides a resistive heater circuit. The layered heater also includes leads that connect the resistive heater circuit to an electrical power source, which is typically cycled by a temperature controller. The lead-to-resistive circuit interface is also typically protected both mechanically and electrically from extraneous contact by providing strain relief and electrical isolation through a protective layer. Accordingly, layered heaters are highly customizable for a variety of heating applications.
Layered heaters may be “thick” film, “thin” film, or “thermally sprayed,” among others, wherein the primary difference between these types of layered heaters is the method in which the layers are formed. For example, the layers for thick film heaters are typically formed using processes such as screen printing, decal application, or film dispensing heads, among others. The layers for thin film heaters are typically formed using deposition processes such as ion plating, sputtering, chemical vapor deposition (CVD), and physical vapor deposition (PVD), among others. Yet another series of processes distinct from thin and thick film techniques are those known as thermal spraying processes, which may include by way of example flame spraying, plasma spraying, wire arc spraying, and HVOF (High Velocity Oxygen Fuel), among others.
Thermally sprayed layered heaters are generally formed by spraying molten powder or wire feedstock onto a substrate in the requisite layers as set forth above. The molten material impacts the substrate, or layers that have previously been applied over the substrate, resulting in lenticular or lamellar grain structure from the rapid solidification of small globules, flattened from striking a cooler surface at relatively high velocities. Due to this resultant grain structure, a common characteristic of thermally sprayed heaters is porosity, which can be beneficial in terms of fracture toughness, but also detrimental in terms of moisture absorption. In heater applications, if the thermally sprayed layers absorb an excessive amount of moisture, this moisture can cause the heater to fail during operation by mechanisms such as delaminating the individual layers or interrupting the supply of electrical power, or enabling excessive leakage current to ground.
Moisture absorption is also an issue in many other types of resistance heaters, and thus improved devices and methods to counteract the adverse affects of moisture on heater performance are continually appreciated in the field of resistance heaters.